MOLLUSC A. 321 



in cases when some of the vessels open out into large spaces, which 

 surround some of the viscera and form a true perivisceral cavity, 

 there is no communication with the coelom, or Avith any other 

 organ, or with the exterior. The so-called aquiferous canals, when 

 present, do not lead into the blood system, but merely into spaces 

 in the integument. 



The heart consists of a median ventricle, and of two lateral 

 auricles (four in Nautilus, one in most Gastropoda) ; it is placed 

 on the dorsal surface in the pericardium (except in Anomia and the 

 Octopoda). As already hinted the ventricle is merely a portion 

 of a longitudinal dorsal vessel (the rest being formed by the aortae) 

 comparable to the dorsal vessel of Annelids. 



The arteries branch, but, except in Cephalopoda, in which capil- 

 laries are found, they end in sinuses amongst the organs, from 

 which the blood passes into the large veins ; these conduct the 

 blood to the gills, part or all of it passing through the kidney on 

 the way. In many cases the blood in the mantle passes directly 

 to the heart without traversing the gills. 



The blood is either colourless or of a slightly blue tinge, due to 

 the presence of haemocyanin (an albuminoid pigment containing 

 copper). In rare cases it is red owing to the presence of haemo- 

 globin in the plasma (Planorbis), or in the corpuscles (Solenogastres, 

 some Lamellibranchs). 



A lymphatic gland consisting of a framework of connective 

 tissue, in which blood corpuscles are formed, is often present on 

 the course of the aorta. 



Respiration is in all cases carried on through the general outer 

 surface of the body, but in addition special respiratory organs in 

 the form of branchiae or of lungs are generally present. 



The branchiae are projections (often ciliated) of the body surface 

 and are usually placed in the mantle-cavity. Very generally they 

 have a certain form designated by the term Ctenidium. A ctenidiuni 

 is a bronchia consisting of an axis attached to the body and bearing 

 two rows of projecting lamellae. Although not always found, the 

 ctenidiuni is the typical Molluscan gill, and is always contained 

 to the number of one, two, or four (Nautilus) in the mantle-cavity. 

 In the Ckitonidae only are the ctenidia more numerous than four. 

 The gills may have other forms (Nudibrancliiata), and in the 

 terrestrial forms are absent altogether, the mantle-cavity having 

 assumed a form and structure suitable for aerial respiration. 



The excretory organs, often called the organs of Bojanus, are 



Y 



